This invention relates to a self-checking circuit for checking a semiconductor sensor such as a stress sensor, an accelerometer, or the like employing piezo resistors.
In an article entitled "Toyota Air Bag Sensor" by M. Mutoh et al., page 95, in Isata, No. 911270 published 1991, there is described a semiconductor sensor checking circuit used with a semiconductor accelerometer sensor formed in a semiconductor substrate having a cantilever. The semiconductor sensor includes piezo resistors formed in the cantilever connected in a bridge circuit. The bridge circuit operates on a bridge driving voltage for producing an output signal across its two output terminals. The output signal corresponds to a stress produced in the cantilever. In order to cause the bridge circuit to produce a pseudo output signal from the bridge circuit for self-checking, the conventional checking circuit employs a source of small current to supply a small current to one of the output terminals of the bridge circuit. A switching device is provided to connect this current source to the one output terminal of the bridge circuit in response to a checking signal.
With such a conventional checking circuit, however, the current source should be selected to provide a very small current. Assuming now that the piezo resistors have a resistance of 5K.OMEGA., the bridge driving voltage is 3 volts, the maximum change of the piezo-resistors is 0.1% of their original values, which corresponds to the dynamic range of the bridge circuit and the output signal corresponds to 10% of the full range of the bridge circuit, the small current required to produce a pseudo output signal should be as small as 30 nA. For this reason, the conventional checking circuit requires current mirrors arranged in a number of stages. This results in a complex, expensive and space-consuming checking circuit.